Rep. Jesse Jackson Jr. resigned from Congress on Wednesday for health reasons, but acknowledged an ongoing federal investigation and vowed to "accept responsibility for my mistakes."

Jackson's decision ends a once-promising political career for the Illinois Democrat, who first came to Washington in 1995 to serve a Chicago-area district.

"For 17 years I have given 100% of my time, energy and life to public service," Jackson wrote in a letter to House Speaker John Boehner. "However, over the past several months, as my health has deteriorated, my ability to serve the constituents of my district has continued to diminish."

Jackson, the eldest son of the civil rights leader, has been on medical leave since mid-June and has been treated twice at the Mayo Clinic for bipolar disorder.

At the same time, Jackson was the subject of two separate investigations of wrongdoing.

The U.S. Justice Department was investigating allegations that Jackson, 47, misused campaign funds to redecorate his house and buy an expensive watch for a friend.

Jackson recently hired former federal prosecutor Dan Webb to work out a plea deal on the campaign-finance allegations. CBS News reported that the deal would allow Jackson to resign for health reasons, serve some jail time, plus require him to repay any campaign funds used for personal purposes. The report also said he would be allowed to keep his congressional pension.

Jackson also has been the subject of a long-running House Ethics Committee investigation stemming from allegations that he raised money for then-Illinois governor Rod Blagojevich in exchange for appointment to the U.S. Senate in President Obama's old seat. Jackson denied wrongdoing in that investigation, which will now end. The panel does not retain authority over former House members.

"He did have a very promising career," said Dick Simpson, a political science professor at the University of Illinois-Chicago and a former Chicago alderman. "He seemed to be on an upward trajectory. He'd had some setbacks ... but nonetheless he was still a very dominant, very important figure in Chicago politics and had the potential to play a role nationally, particularly if the Democrats ever took back control in the House."

In his letter to Boehner, Jackson said he had made his share of "mistakes."

"I am aware of the ongoing federal investigation into my activities and I am doing my best to address the situation responsibly, cooperate with the investigators, and accept responsibility for my mistakes, for they are my mistakes and mine alone," Jackson said.

House Minority Leader Nancy Pelosi, D-Calif., praised Jackson's congressional service and said it was "marked by his eloquent advocacy for his constituents' views and interests."

Jackson's resignation will spark a special election in his Chicago-area House seat, a heavily Democratic enclave. Illinois state law requires that an election must be held within 115 days after the seat is vacated.

Illinois Gov. Pat Quinn, a Democrat, said in a statement that he will set an election schedule within the next five days. "This election will be carried out in a manner that is fair to the electorate and as economical as possible for taxpayers," Quinn said.

Local news reports have said possible Democratic candidates in the 2nd District include Cook County Chief Administrative Officer Robin Kelly, state Sen. Toi Hutchinson and state Sen. Kwame Raoul, who holds the legislative seat once held by Obama.

Unless Jackson is convicted of a felony that would prevent him from collecting his congressional pension, he would be entitled to about $45,000 in annual benefits when he turns 62, according to Pete Sepp, executive vice president of the National Taxpayers Union.